FallbackProvider with event 'noNetwork'

[pofulu]

I'd like to use the FallbackProvider to handle rpcUrl that could stop.
But it seem that FallbackProvider won't work when any of provided rpcUrl is unreachable.

const provider = new FallbackProvider([
  new JsonRpcProvider('http://127.0.0.1:8545'),
  new JsonRpcProvider('http://127.0.0.1:1234'), // not exist
], 1);

I expect it will work because the quorum is 1, and the first url http://127.0.0.1:8545 works. But I got the noNetwork error:
Error: could not detect network (event="noNetwork", code=NETWORK_ERROR, version=providers/5.6.8)

Maybe I misunderstood how FallbackProvider works, is there any idea for this case?